1. Field of the Invention
The present invention relates to an antenna device, and more particularly to a technique for improving the radiation efficiency of an antenna device.
2. Description of the Related Art
An antenna element in a small-sized wireless terminal such as a portable telephone device is required to have a small size and high radiation efficiency. To satisfy both requirements, various efforts have been made to find the material suitable as a dielectric body forming the antenna element and determine the appropriate shape of a radiation electrode (see Japanese Patent Application Laid-Open Nos. 2001-53528 and 2007-74585).
In the case of a portable telephone device that is often carried around by the owner wherever he/she goes, the portable telephone device falling from the height of a breast pocket or the like onto the ground is a conceivable accident. In this case, a housing of the portable telephone device might be temporarily deformed, and the impact might be transmitted to the antenna element inside. With the resistance to impact applied from the outside (hereinafter referred to as the “impact resistance”) being taken into consideration, a void (a margin) of approximately 1 mm is left between an antenna element and a housing, in general.
The inventor observed that the void for impact resistance might decrease the radiation efficiency of the antenna. The present invention has been developed based on the technical findings obtained through studies made on the relationship between the void and the radiation efficiency.
The present invention has been completed, with the above consideration by the inventor being the starting point. The main object of the present invention is to improve the radiation efficiency of a built-in antenna element.